Wednesday, July 13, 2016

Implications of an Amplified July Pattern

The large-scale circulation of the atmosphere undergoes substantial seasonal changes, especially in the Northern Hemisphere, home to a majority (68%) of our planet's land areas.

The analysis below shows the average 500-mb heights over North America in July (the flow parallels the height contours with lower heights on the left).  During July, the storm track is displaced northward compared to its wintertime position and hugs the US–Canadian border.  Upper-level ridging lies over the Atlantic Ocean near 30ºC and extends westward over central North America, centered roughly on Texas.  This leads to easterly upper-level flow in the subtropics over the Caribbean, Gulf of Mexico, Mexico, and the subtropical east Pacific.  It is the development of this easterly flow during the summer, which contrasts with the westerlies found during winter over much of this region, that is characteristics of the  North American Monsoon system and a key reason for a pronounced summer peak in rainfall over Mexico and the southwest United States.
The latest GFS forecast for 1800 UTC 16 July (Noon MDT Saturday) shows a fairly typical July pattern.  Upper-level ridging extends across the eastern Atlantic near 30ºN and central North America.  Easterlies predominate to the south over the tropics and subtropics (note the active tropical storm track in the subtropical eastern Pacific) with the midlatitude storm track perhaps just a bit south of climatology over the northern continental US.  There's also a weak trough along the west coast, similar to climatology, albeit shifted slightly inland.

That's all fine and dandy, but going out a bit further to 1200 UTC 18 July (6 AM MDT Monday) shows an amplification of the overall pattern with the west coast trough deepening and the ridging over central North America strengthening and pushing northward.

Looking further out at the gold-standard ECMWF ensemble we see a monster ridge in firm control over central North America by 0000 UTC 21 July (6 PM MDT Wednesday).

Source: ECMWF
All of this is bad news for the central US. as it will likely mean a major heat wave next week.  The CPC 6–10 day outlook pegs a > 90% chance of above average temperatures over Nebraska and Kansas.  Temperatures in and around those states are likely to be well above average.

Source: CPC
There are a host of reasons why they call the central US the fly-over states, and next week will certainly be a good time to fly over them.

You will notice that Utah is in the transition zone, making our forecast a bit tricky.  A best-case scenario for us is for the overall pattern to be shifted eastward, putting us more under the influence of the large-scale trough.  If, however, the pattern is shifted westward, with the ridge bulging over the Intermountain West, we could be seeing some heat as well, unless we can get a monsoon trajectory going to push in some moisture.  It's too soon to say for us, but the central US looks to get cooked.

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